In Dragon Age Origins, you begin by choosing one of three character classes and begin your ‘origin’ type story before getting into the main quest. The classes are warrior, mage, and rogue. You also have different races to choose from, including humans and elves, and drilling down even further you have different race types. The idea is that you can start the game over many times and get a little something different each time. Although this game is not set in the Dungeons & Dragons world, it bears many similarities. In all fairness, D&D borrowed very heavily from J.R.R. Tolkien, and pretty much every fantasy book, game, or movie ever made after that borrowed heavily from him.
Once your origin story is out of the way and you get into the main plot of the game, you will become a Grey Warden. They are like a secret order of knights sworn to protect the land. When a huge invasion leaves the king dead and a villainous power-hungry knight trying to usurp the throne, you find yourself on the losing side of a civil war. It’s up to you and the last remaining Wardens to help restore peace to the land, and you’ll be outnumbered all the way to the end.
Once you've completed the main story, you can buy the expansion back called Dragon Age: Awakening and add many more hours of gameplay.